Steplessly variable transmissions are known incorporating V-belt drives, and having variable-pitch pulleys, such as described in my U.S. Pat. Nos. 3,699,827 and 4,026,161. These transmissions incorporate means for pressing the two disks of flanges of the pulleys together with a force dependent on the torque transmitted between the two of them by the flexible member constituted as a V-belt.
It is also known, as the example of the German patent publication Nos. 1,909,887 and 2,012,732 as well as from U.S. Pat. No. 3,044,315 to use tandem belts in such a variable-speed transmission. To this end each of the pulleys or sheaves is constituted by a pair of flanges or disks one of which is movable relative to the other, and by an intermediate disk placed between them. The intermediate disk forms with each of the outer disks a respective nip and the tandem belts are seated in these nips. Such a system is used to transmit considerably greater torque than can normally be transmitted by a single-belt transmission.
The main difficulty with these tandem-belt arrangements is that they are relatively complex. The various disks of the pulleys must be rotationally coupled together, yet able to move axially relative to each other with a high degree of freedom. At the same time the torque-responsive mechanism must be able to operate acurately on both of the belts, normally through the intermediary of one of the disks. A particular problem with such an arrangement is that the intermediate disk becomes fixed relative to one of the outer disks thereby eliminating the variability of the transmission. This typically occurs when the complex guiding structure for the intermediate disk on the outer disks rusts or becomes foul.